InputSlot heuristic for photo sizes doesn't always work

[yetamrra]

In _ppdCacheGetInputSlot(), there's a check that tries to look up the value for "photo" if media-source wasn't supplied in the request. This seems to work fine for a lot of printers, but breaks in this scenario:

1. Japanese printer has an L-sized photo tray.
2. User loads Hagaki paper in the main tray and tries to print without specifying a tray.
3. cupsd sees that Hagaki is smaller than 5x7" and automatically requests the photo tray.
4. Printer fails the job because Hagaki can't be loaded from the L-sized photo tray.

Should this check change to look up "auto" instead? Based on our PPD database, removing it entirely would break a few models, but "auto" should work in nearly every case. With a small refactor, it could also keep the "photo" case as a fallback for PPDs that have a photo InputSlot option but not an auto option.

[michaelrsweet]

I like the idea of defaulting to "auto" when no media-source is supplied and only mapping when there is no auto source.

[yetamrra]

OK, I'll put together a PR later today.